Jacuzzi pool heater attempts to start but fails; display shows F3 and no flame is established.

Safety first: Turn off power and shut off gas at the manual gas valve before removing any panels. If you smell gas, evacuate the area and contact your gas supplier or emergency services.

Steps to troubleshoot F3:

• Verify gas supply: Ensure the main gas valve to the Jacuzzi heater is open. Confirm other gas appliances on the same line operate normally.
• Check for ignition attempt: With panels in place and gas on, restore power and call for heat. Observe through the sight glass: do you see the igniter glowing (hot surface) or hear/see sparking (spark ignition)? If not, suspect the igniter or ignition control.
• Inspect igniter: With power off, remove the igniter assembly. Look for cracks, heavy oxidation, or broken elements. Measure resistance with a multimeter; an open circuit or out-of-spec reading indicates replacement is needed.
• Inspect wiring and ground: Check all harness connections between the ignition control board, gas valve, and igniter. Ensure the heater chassis is properly grounded.
• Gas valve check (professional): If the igniter operates but there is no flame and no gas odor at the exhaust, the gas valve may not be opening. This should be tested and replaced only by a licensed gas technician.

Do not repeatedly reset and attempt ignition if F3 persists; have a qualified Jacuzzi service technician diagnose the gas train and ignition control.